For a transmission system, as one network connection service offered to users by a communication common carrier, a guaranteed bandwidth service enables multiple users to use a network of the communication common carrier, at a guaranteed constant communication bandwidth established based on a contractual agreement between the communication common carrier and each user. The guaranteed bandwidth service is used when, for example, communication is of high importance and requires high reliability. The guaranteed bandwidth service is used for, for example, virtual private network (VPN) connections of a corporate user and backhaul connection between a wireless base station and a core network of a mobile communication common carrier.
The guaranteed bandwidth service has been disclosed as an example by the Metro Ethernet Forum that promotes the use of Ethernet (a registered trademark) technology in metropolitan area networks (MAN), by providing wide-area Ethernet services, creating technical specifications, etc., (see, for example, Metro Ethernet Forum, “Introducing the Specifications of the MEF, An Overview of MEF 6.1 and MEF 10.1”, pp. 35-36, [online], [retrieved on Dec. 24, 2010], from the Internet; and Metro Ethernet Forum, “MEF Technical Specification MEF10.2, Ethernet Services Attributes Phase 2”, p. 37, “7.11.1 Standard Bandwidth Profile Parameters and Algorithm”, [online], [retrieved on Dec. 24, 2010], from the Internet).
For example, according to the technical specification prescribed by the Metro Ethernet Forum, bandwidth parameters used when an Ethernet virtual connection service is provided in a wide-area Ethernet network, are defined that include a committed information rate (CIR) meaning a guaranteed bandwidth and an excess information rate (EIR) meaning an upper limit bandwidth available for transmission when unused bandwidth is present though no bandwidth is guaranteed. “Ethernet” is a registered trademark.
Not only wired links via metal cables and optical fiber cables but also wireless links using microwaves are used as connection links in the network of a communication common carrier that provides a guaranteed bandwidth service. For wireless links, control may be executed according to wireless channel quality, where a high speed modulation scheme (for example, 64 QAM, 128 QAM, or 256 QAM) is used for quality that is high, and a low speed modulation scheme that is highly error resistant (for example, BPSK or QPSK) is used for quality that is low. Such control is referred to as, for example, “adaptive modulation”. Further for wireless links, transmission speed also varies consequent to adaptive modulation.
For a guaranteed bandwidth service, it is considered that the user enters a contractual agreement stipulating a guaranteed rate as an item of the service level agreement (SLA) to be satisfied. On the other hand, a peak rate is not subject to the SLA and therefore, in a case where traffic of the guaranteed rate can be transferred, no violation of the service agreement can be established even if traffic of a volume up to the peak rate is not transferred. On the other hand, it is more comfortable for the user to be able to communicate at a higher throughput and therefore, it is desirable that traffic of a volume up to the peak rate can be transferred.
Link aggregation is known that is formed by aggregating multiple communication links and that is used as a high capacity link, to realize higher data transfer (see, for example, Japanese Laid-Open Patent Publication Nos. 2007-060494, 2005-252333, and 2004-080139; and IEEE Std 802.3ad-2000, 2000/03/30, “Amendment to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications”, [online], [retrieved on Dec. 24, 2010], from the Internet; Ishizu, Murakami, and Harada, “Link Aggregation Platform Enabling Flexible Traffic Control in a Cognitive Wireless Networks”, IEICE Technical Committee on RCS, Aug. 27, 2010; and Higuchi, et al., “Analysis of Optimal Traffic Distribution Control in a Heterogeneous Wireless Link Aggregation Method”, IEICE Technical Report, Vol. 109, No. 383, SR 2009-74, pp. 1-8, Jan. 2010). An example of the link aggregation is IEEE 802.3ad as a technical standard employed when link aggregation is formed and used for wired Ethernet links.
According to IEEE 802.3ad, the transmission speeds of all the Ethernet links are equal as a precondition, and no sequence number is given to any data packet and the sequence guarantee of the packets is realized using the data stream (conversation). Therefore, even though multiple Ethernet links are aggregated, the data packets are transmitted using any one of the Ethernet links for each data stream.
However, according to the conventional techniques, a problem arises in that traffic throughput can not be enhanced. For example, the transmission speed of each of the wireless links executing the aggregation varies consequent to, for example, frequency-selective fading and adaptive modulation. In a case where the transmission speed of a wireless link to which data is distributed is lower than a speed commensurate with the amount of data and even if bandwidth in another wireless link is available, a portion of the data is discarded or delayed and therefore, the throughput drops.